Young adult same-sex twins will be run through two psychophysiological testing sessions separated by a 1 week interval. Measures of OR habituation, reducing/augmenting, preception and strength of nervous system will be obtained in both sessions. Dependent variables will include bilateral skin conductance, heart rate, bilateral EEG/ER (C3 and C4), and a measure of stimulus disruption of a tracking task. All subjects have previously completed the Differential Personality Questionnaire, the Activity Preference Questionnaire and will complete a special mood state rating form in each testing session.